I have installed the plugin and once SEF is turned on I get pages loading which do not load CSS or images?

My web server is apache, PHP interface cgi - Fcgi.

Joomla version is 1.5.7 and running version PHP 5.2.6

I have tried to follow a number of threads which explain the renaming of .htaccess but once I rename this I then get a Internal server error 500?

The links have been inserting index.php/index.php/then what ever the link is.html?

If someone could let me know what part of the htaccess file I should change and what to then that would be great.